1. Field of the Invention
The present invention relates to an ignition coil for an internal combustion engine, that is attached to an automobile engine, for example, for applying to an ignition plug provided at the bottom of a plug hole of the engine high voltage for causing the ignition plug to generate spark discharge.
2. Description of the Prior Art
JP-P-A HEI 9-246070, for example, discloses an ignition coil comprising a slender cylindrical main body inserted into a plug hole of an engine and having an open magnetic circuit core at its center, a primary coil and a secondary coil outside the core, an armoring core on its outer periphery and a high-voltage terminal at its tip; and an ignition plug provided at the bottom of the plug hole and having a top terminal that is directly connected to the high-voltage terminal to apply high voltage to the top terminal. In this prior art ignition coil, since the slender cylindrical main body is substantially entirely accommodated in the plug hole, the space required outside of the engine for installing the ignition coil is extremely small.
However, since an engine is generally made of aluminum alloy so as to minimize its weight and the open magnetic circuit core is disposed parallel to the axis of the plug hole, when an electric current is applied to the primary coil and the magnetic flux generated by the electric current flows through the core, an eddy current is generated in the aluminum portion surrounding the plug hole. This eddy current generates a magnetic flux that cancels the magnetic flux generated by the electric current to degrade the efficiency of the ignition coil.
There has recently been increasing demand for a high-output ignition coil for an internal combustion engine, that is applicable to a lean-burn engine or an alternative fuel engine. However, since the prior art ignition coil does not have good efficiency as described above, it does not meet this demand. In addition, since the plug hole for accommodating the ignition coil is inevitably restricted in size from the standpoint of the engine structure, it is difficult to secure sufficient insulation distance within the slender cylindrical main body of the ignition coil accommodated in such a size-restricted plug hole. Furthermore, since the temperature inside the plug hole is high owing to the heat from the engine, the ignition coil accommodated in the plug hole is exposed to a high temperature. Therefore, both the insulation distance and the high temperature should be taken into account in designing the ignition coil.
To eliminate these disadvantages, JP-UM Registration No. 3052284 proposes a molded ignition coil having a closed magnetic circuit as shown in FIG. 3, that comprises a high-voltage terminal 6 installed outside an engine, an ignition plug 40 attached to the bottom of a plug hole 31 of the engine, and a high-voltage relay joint 42 for electrically connecting the high-voltage terminal 6 and the ignition plug 40. In this prior art ignition coil, two E-shaped layer-built cores 3a and 3b constituting the closed magnetic circuit have their respective central legs 3' inserted into the interior of a bottomed insulation casing 4, one from a bottom side and the other from an open top side, so that the central legs 3' abut against each other and are joined together by means of an adhesive agent or welding or by other such means within the interior of the casing 4 and support therearound a cylindrical primary coil 1 and a cylindrical secondary coil 2 concentrically disposed. Opposite side legs 3" of the core 3a abut against and are joined to those 3" of the core 3b, respectively, in the same manner as the central legs 3', but outside the casing 4.
Further, the hole in the bottom 4' of the insulation casing 4, from which the central leg 3' of the core 3a, for example, is inserted, is stopped up by a bobbin for the primary coil 1; and the central leg 3' of the core 3a is thrust from the bottom side into the inner circumference of the primary coil bobbin. On the other hand, the central leg 3' of the core 3b is thrust from the open top side into the inner circumference of the primary coil bobbin and abuts against and is joined to the central leg 3' of the core 3a by means of an adhesive agent or the like means.
The prior art ignition coil is installed on the engine, and high voltage is applied from the ignition coil to the ignition plug 40 provided at the bottom of the plug hole 31. A rocker cover 33 provided with a cylindrical extension hole 32 communicating with the the upper opening of the plug hole 31 is laid over and fixed to the engine. The bottom 4' of the insulation casing 4 of the ignition coil is directed laterally so that the pair of E-shaped cores 3a and 3b with their respective legs joined together are orthogonal to the axis of the plug hole 31 and extension hole 32. In this state, the base portion 3 of one of the E-shaped cores 3a and 3b thrust out of the bottom 4' of the insulation casing 4 and the base portion 3 of the other of the E-shaped cores 3a and 3b thrust out of the open side of the casing 4 are attached by means of bolts etc. to a flange 35 protruding outwardly from a dish-shaped portion 34 integrally formed on the top of the cylindrical extension hole 32 of the rocker cover 33. A high-voltage cylindrical portion 7 integrally extending from the lateral side of the insulation casing 4 for supporting the high-voltage terminal 6 therein is directed downward and thrust into the interior of the extension hole 32 via the interior of the dish-shaped portion 34 of the rock cover 33.
An upper socket 43 of the high-voltage relay joint 42 covers the high-voltage cylindrical portion 7 and stops up the upper opening of extension hole 32, whereas a lower socket 44 of the high-voltage relay joint 42 covers the upper half of the ignition plug 40 provided at the bottom of the plug hole 31. A conductive member, such as a coil spring 45, retained within the high-voltage relay joint 42 in a piercing state electrically connects the high-voltage terminal 6 of the ignition coil and the top terminal 41 of the ignition plug.
One of the base portions 3 of the pair of opposed E-shaped cores 3a and 3b of the ignition plug protrudes outwardly from the bottom side of the insulation casing 4, and the other thereof protrudes outwardly from the open side of the insulation casing 4, that is stopped up by a solid layer 5 of insulating resin. When the insulation casing 4 is laid lateral to make the cores 3a and 3b orthogonal to the axis of the plug hole 31, the distance between the base portions 3 of the cores 3a and 3b becomes long. Since the base portions 3 are mounted on the flange 35 of the dish-shaped portion 34, the size of the dish-shaped cover 34 of the rocker cover 33 fixed outside the engine becomes correspondingly large. This requires a large space outside the engine for the installation of the dish-shaped cover 34.
One object of the present invention is to provide a small-sized ignition coil that does not require a large space outside an engine for the installation thereof and can suppress output decrease caused by occurrence of an undesirable magnetic flux.